1. Field of the Invention
The present invention relates to a sigma-delta modulated amplifier operable to amplify an input signal. More particularly, the invention relates to a sigma-delta modulated audio amplifier operable to sum, integrate, quantize, sample, and amplify an input signal.
2. Description of the Related Art
It is often desirable to amplify audio signals using switch-mode audio amplifiers, often referred to as Class-D amplifiers. The general design of known switch-mode audio amplifiers is substantially similar to that of linear amplifiers, such as Class A, B, and AB amplifiers, with a difference being the nature of signals provided to output stages. Rather than feeding an audio waveform directly to the output stage, as is done in linear amplifiers, the switch-mode amplifier feeds an audio waveform into a Pulse Width Modulator (PWM) circuit which then feeds modulated pulses to an output stage for amplification. By quickly switching the output stage completely on and completely off with varying pulse widths, the switch-mode amplifier is able to recreate waveforms of almost any shape, and, by filtering the switching output, sound is produced by a speaker, such as a loudspeaker, connected thereto. Generally, the pulses are fed to the output stages at a frequency between 200 and 500 kHz, or 200 to 500 thousand pulses per second, which is often desirable to produce a smooth waveform for the loudspeaker.
An advantage of switch-mode amplifiers is that the output stage transistors are switched either completely on or completely off. Amplifier topologies that operate in a partially on state, such as Class A and AB amplifiers, act like resistors and produce heat, thereby wasting energy even during periods of non-amplification. Thus, switch-mode amplifiers are substantially more efficient than non-switching linear amplifiers. Higher efficiency and less heat generation allows switch-mode amplifiers to utilize smaller power supplies and to be offered in more compact packages than comparable linear amplifiers.
Existing switch-mode audio amplifiers generally incorporate one of two modulator topologies; carrier-based modulators; or integrating modulators with one or more feedback loops, typically referred to as sigma-delta modulators. Carrier based modulators generate PWM signals by comparing a reference carrier waveform, typically a triangle or saw waveform, to an audio signal using one or more comparators. The performance of carrier-based amplifiers is greatly dependent on the linearity and noise performance of the carrier oscillator. Integrating modulators, or sigma-delta modulators, typically feed the output switching voltage waveform back to the input wherein the signal is summed and integrated with the audio signal. Quantizing the integrated output creates PWM signals for switch-mode amplification.
Unfortunately, existing switch-mode audio amplifiers, including amplifiers having sigma-delta modulators, suffer several disadvantages such as electromagnetic interference (EMI), high-frequency instability, spectral purity, and costly complex integrated components. Existing switch-mode audio amplifiers typically operate in a fixed frequency mode wherein the modulator and output transistors oscillate at a fixed frequency. Thus, these circuits produce significant energy levels at the carrier frequency and the associated carrier harmonics. Energy at these frequencies is undesirable as it may prohibit compliance with regulatory standards in both conducted and radiated EMI. Therefore, existing designs must employ complex and costly filtering methods to reduce such EMI energy levels in order to comply with regulatory standards. These complex and costly filtering methods often require AC mains power filtering, extensive RF shielding, and multi-pole amplifier output filtration. Such output filtration significantly degrades the audio performance (harmonic and intermodulated distortion) of the amplifier.
Furthermore, existing switch-mode amplifiers that employ spread spectrum modulation attempt to randomize the switching frequency in an effort to reduce EMI energy levels. In the case of carrier-based modulators, such randomization requires a complex, frequency-agile triangle wave oscillator that adds significant cost and design complexity. Such complex frequency-agile triangle oscillators suffer from non-linearities and high frequency noise that result in poor spectral purity (i.e. increased harmonic and intermodulated distortion).
In the case of low-order sigma-delta modulators (equal-to or less-than 3rd order), complex dynamically controlled digital delay lines may be incorporated to randomize the switching frequency, shown in FIG. 1C, or complex continuous-time analog randomization signals may be added into the signal path prior to quantization, shown in FIG. 1B. Referring to FIG. 1D, in the case of high-order sigma-delta modulators (greater than 3rd order), all of which are costly fully integrated solutions, complex multi-loop circuits may be incorporated to result in randomized idling patterns, thus randomizing the switching frequency. Such implementations are generally undesirable as they increase design complexity and cost.
Similarly, existing switch-mode amplifiers that employ a sigma-delta modulator with one or more feedback loops typically operate in one of two modes, self-oscillation (FIG. 1E) or over-sampled clocked quantization (FIG. 1A). Self-oscillation techniques suffer from limited frequency control and are typically operated in a fixed-frequency mode thereby increasing switching harmonics and EMI energy. Over-sampled clocked quantization modulators suffer the effects of sampling continuous-time analog signals with complex analog sampling circuitry, i.e. significant setup and hold times and increased susceptibility to unwanted harmonics. Additionally, over-sampled clocked quantization modulators have non-randomized quantizer clocks and sample rates, thereby further increasing design complexity. Similarly, such designs increase EMI and require additionally output filtering, thereby increasing distortion.
Existing switch-mode amplifiers that employ a sigma-delta modulator with one or more feedback loops typically control bandwidth and voltage level of the loop return by low-pass filtering and resistive division. Unfortunately, such sigma-delta modulators suffer from high-frequency stability problems that, when stimulated with high-frequency audio signals, results in undesirable high-frequency harmonic content. Efforts have been made to enhance loop stability through higher-order modulators requiring numerous feedback loops. Such high-order modulators are complex, costly, and therefore, implemented only in generally undesirable integrated solutions.
Another problem common to all switch-mode power amplifiers employing two or more transistors is shoot-through prevention circuitry. Such circuits typically add a dynamically controlled dead time between hi-side and low-side transistor conduction. These circuits are a fundamental source of distortion in the amplifier and often times are constructed with specialized integrated controllers that add design cost and complexity. Additionally, matching the propagation delays for high-side and low-side transistor control and drive signals becomes critical. Mismatched delays between transistors, and the associated modulator control signals, require additional dead time to accommodate the mismatch, typically in excess of 80 ns, thus increasing distortion.
Existing switch-mode amplifiers that employ a sigma-delta modulator fail to incorporated complete electrical isolation, AC and DC, within the modulator, therefore requiring complex and potentially large, heavy, isolated power supplies. Furthermore, because the significant portion of any incoming power is required to drive the output stage and the loudspeakers connected thereto, a power supply isolating the output stage must be substantially larger than a power supply isolating the input stage. Even in applications incorporating sigma-delta modulators where the outputs are not user-accessible, no effort is typically made to isolate the input stage from the output stage. Where input-to-output isolation is attempted, audio transformers are typically used. Unfortunately, these transformers suffer from limited frequency response, making implementation difficult.